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S.A. Macko

Publications and source records attributed to S.A. Macko.

5 recordsLinked to original sources
USGS2007

Preliminary identification of ground-water nitrate sources using nitrogen and carbon stable isotopes, Kansas

Increasing nitrate-N in ground water is a problem in areas with limited ground-water supplies, such as central Kansas. Nitrate-N concentrations in ground water in the study area in Ellis County range from 0.9 to 26 mg/L. Calculated mean values observed in soil cores are 1.2-15 mg/kg. The ??15N signatures of the ground waters are more enriched (+16.8 to +28.7???) than those of the soils (+8.4 to +1 3.7???), strongly suggesting that nitrate-N sources are not from mineralized and labile nitrogen present in the unsaturated zone. Soil cores were collected near municipal wells to determine if soil nitrogen was a contributing source to the ground water. Increased ??15N of total nitrogen with depth suggests that microbial mineralization processes and possible denitrification or volatilization isotope enrichments have affected the observed ?? 15N signatures in the soil. However, the observed soil-nitrogen values are not of sufficient magnitude to explain the nitrate-N concentrations or associated ??15N values observed in the ground water. Stable carbon isotopes provide some supporting evidence that soils are not a major contributor to the observed nitrate-N concentration in the ground water. ?? 13C values of the dissolved organic carbon (DOC) in soils generally become more enriched with depth while corresponding ground-water ??13C (DOC) values are more depleted than in the overlying soils. Carbon isotope values of the soils are indicative of a C4 plant source that is enriched by microbial processes. The ??13C (DOC) of ground water indicates C3 values that may reflect impacts from animal-waste sources.

Current Research in Earth Sciences↗
USGS2001

Diets of nesting laughing gulls (Larus atricilla) at the Virginia Coast Reserve: observations from stable isotope analysis

Food web studies often ignore details of temporal, spatial, and intrapopulation dietary variation in top-level consumers. In this study, intrapopulation dietary variation of a dominant carnivore, the Laughing Gull ( Larus atricilla ), was examined using carbon, nitrogen, and sulfur isotope analysis of gull tissues as well as their prey (fish, invertebrates, and insects) from the Virginia Coast Reserve estuarine system. As earlier traditional diet studies found evidence of individual dietary specialization within gull populations, this study used stable isotope analysis to assess specialization in a coastal Laughing Gull population. Specifically, blood, muscle, and feather isotope values indicated significant intrapopulation dietary specialization. Some gulls relied more heavily on estuarine prey (mean blood δ 13 C = -17.5, δ 15 N = 12.6, and δ 34 S = 9.3), whereas others appeared to consume more foods of marine origin (mean blood δ 13 C = -19.4, δ 15 N = 14.8, and δ 34 S = 10.4). It is important to account for such dietary variability when assessing trophic linkages in dynamic estuarine systems.

Isotopes in Environmental and Health Studies↗
USGS2001

Stable-isotope analysis of canvasback winter diet in upper Chesapeake Bay

A major decline in submerged aquatic vegetation (SAV) in Chesapeake Bay has altered the diet of wintering Canvasbacks ( Aythya valisineria ) from historically plant to a combination of benthic animal foods, especially the ubiquitous Baltic clam ( Macoma balthica ), supplemented with anthropogenic corn ( Zea mays ). Because the isotopic signature of corn is readily discriminated from bay benthos, but not SAV, we used stable-isotope methodology to investigate the corn–SAV component of the winter diet of Canvasbacks. Feeding trials with penned Canvasbacks were conducted to establish turnover rates and fractionation end-point loci of δ 13 C and δ 15 N signatures of whole blood for individual ducks fed ad libitum diets of (1) Baltic clams, (2) Baltic clams and corn, and (3) tubers of wild celery ( Vallisneria americana ). Turnover time constants averaged 4.5 weeks, indicating that signatures of wild ducks would be representative of bay diets by late February. Isotopic signatures of wild Canvasbacks sampled in February fell on a continuum between end-point loci for the Baltic clam and the combination Baltic clam and corn diet. Although that finding verifies a clear dependence on corn–SAV for wintering Canvasbacks, it also reveals that not enough corn–SAV is available to establish ad libitum consumption for the 15,000+ Canvasbacks wintering in the upper bay. On the basis of mean δ 13 C signature of bay Canvasbacks ( n = 59) and ingestion rates from feeding trials, we estimated that 258 kg corn per day would account for the observed δ 13 C enrichment and supply 18% of daily energetic needs for 15,000 Canvasbacks. That level of corn availability is so realistic that we conclude that SAV is likely of little dietary importance to Canvasbacks in that portion of the bay.

The Auk↗